The exhaust plume of gas turbine engines, e.g., military aircraft engines, is a source of high infrared energy which may be used for targeting/tracking purposes by heat seeking missiles and/or various forms of infrared imaging systems, e.g., Night Vision Goggles (NVGs). With respect to the former, a heat seeking missile obtains directional cues from the infrared energy wherein the intensity, e.g., temperature, thereof is determinative of the accuracy, and consequently, lethality of the heat seeking missile. Regarding the latter, infrared imaging systems amplify the infrared energy by a factor of about 10,000 and may be used by hostile forces for early detection and/or targeting of surface-to-air/air-to-air missiles. Accordingly, it is highly desirable to reduce the infrared emissions (also referred to as the IR signature) of such engines to a practical minimum to avoid detection, acquisition and/or tracking by enemy threats/forces.
Various infrared suppression systems have been designed and fielded which effect mixing of low temperature ambient air with the high temperature engine exhaust to reduce the IR signature radiated therefrom. Generally, it is the principle objectives of such systems to: (a) reduce the infrared energy below a threshold level (e.g., a level capable of being sensed by the perceived threat), (b) maintain engine performance, and, (c) minimize the weight penalties associated therewith. Secondary objectives may include: (i) minimizing system or configuration complexity to reduce fabrication costs, (ii) minimizing the external aerodynamic drag produced by such IR suppressors, and/or (iii) suppressing the acoustic emissions emanating therefrom which may also be a source of detection.
Amelio U.S. Pat. No. 5,699,965 describes an infrared suppressor for a gas turbine engine which employs a high aspect ratio duct/nozzle to produce a "thin film" or sheet of engine exhaust. The nozzle is disposed in combination with a mixing duct for pumping and, consequently, mixing cool ambient air with the high temperature engine exhaust. The thin film of engine exhaust produces a large shear or surface area for improving the efficacy of mixing and, consequently, the degree of IR suppression. Optionally, Amelio discloses a means for rotating the nozzle/duct so as to change the "line-of-sight" relative to radar scanning/heat seeking threats and/or to change the direction of the exhaust flowing therefrom. With respect to the latter, it is oftentimes desirable to prevent impingement of hot engine exhaust on adjacent structure, e.g., an aircraft fuselage or vehicle skin, so as to avoid creating another "hot spot" for detection, i.e., in addition to the primary source associated with the nozzle/exhaust plume. By rotating the duct, Amelio controls the direction of the engine exhaust and its interaction with other external influences, e.g., the downwash of a helicopter main rotor, to reduce the overall IR signature of the aircraft/vehicle.
While the teachings of Amelio significantly reduce the IR signature radiated from the engine, such IR suppressors have certain drawbacks and limitations. Firstly, and perhaps most significantly, such IR suppressors are limited by the envelope restrictions of a particular application. That is, the elongate manifold thereof, which may require as much as twelve (12) feet to provide ample flow area for the engine exhaust, is not readily adaptable to smaller, more compact, aircraft/vehicles. Secondly, the efficiency of such IR suppressors is limited by the surrounding flow field. That is, the pumping action of such IR suppressors is easily stalled/disrupted by relatively small crossflow disturbances of ambient air, e.g., rotor downwash. Thirdly, when considering the primary embodiment of Amelio wherein the IR Suppressor is fixed/non-rotating with respect to a helicopter/aircraft fuselage, it will be appreciated that, for a particular roll attitude, the entire nozzle is vulnerable to IR scanning radar. That is, since the nozzle is linear there exists one angle wherein the fill length of the nozzle is viewable by IR scanning devices. Accordingly, it is at this angle that the IR signature is maximum. Finally, when considering the alternate embodiment wherein the IR suppressor is rotatable, it will be appreciated that the inclusion of actuators to rotate the duct/nozzle may be prohibitive in terms of weight, present reliability issues, and pose packaging difficulties.
A need, therefore, exits for providing an Infrared Suppression System which is compact in design, rapidly and thoroughly diffuses the IR energy emitted/radiated from a gas turbine engine, provides efficient mixing/pumping irrespective the crossflow disturbances of the surrounding flow field, minimizes impingement of engine exhaust onto adjacent structure, and reduces the overall IR signature of the aircraft/vehicle for a given viewing/azimuth angle.